Electronic applications such as Printed Wiring Boards (PWBs) operating with radio frequencies at or above the GHz range are driving the need for smooth copper features. This need is due to the well-documented “skin effect,” where the electrical signals tend to take a path towards the outer surface of conductors as the frequency is increased. Therefore as frequency is increased, roughness on the surface of the copper can result in significantly higher resistive losses and longer effective line length, both of which contribute to higher conductive losses for the signal. Similarly, high frequency signal losses can also be affected by roughness in the voltage or ground plane that is referenced by the signal.
Unfortunately, conventional PWB processes are diametrically opposed to providing smooth copper surfaces. Conventional PWB processes purposely roughen the copper surfaces in order to provide adequate copper-to-laminate adhesion within the composite structure. The need for a bondable copper surface applies to all copper surfaces, including the inside surface of the foil used to make the initial copper-laminate cores, as well as the final signal or power artwork that is etched or plated on the core during circuitization. This roughening is normally part of an overall treatment that includes the introduction of organics to protect the copper and to promote adhesion. The overall goal of the roughening treatment is to produce a bondable copper surface (i.e., one that has reasonably good bond strength for the resin/laminate system).
The problem encountered with emerging high frequency applications is that the roughening treatment, while critical to the mechanical integrity of the structure, can greatly limit electrical performance, or force the use of relatively wide circuit lines which can greatly limit wiring density.
Adhesion promotion over smooth copper can be useful for other applications besides high frequency signal circuits. For example, a core comprising tightly spaced voltage planes surrounding an insulating material can be used for buried capacitance. In this case, the use of smooth copper on the interior of the voltage planes can facilitate tighter plate to plate spacing, and therefore higher capacitance, as well as better yield and reliability than is possible in a core constructed with interior copper surfaces that must be roughened.